<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        body {
            background: url(images/1.jpg);
            -webkit-background-size: cover;
            background-size: cover;
        }
    </style>
</head>
<body>
<select name="" id="sele">
    <option value="1">春意绵绵</option>
    <option value="2">夏日炎炎</option>
    <option value="3">秋风瑟瑟</option>
    <option value="4">冬雪皑皑</option>
</select>
</body>
</html>
<script>
    var osele=document.getElementById('sele');
    osele.onchange=function(){
        var opt=this.options;
        console.log(this.value);
        switch(this.value){
            case "1":
                document.body.style.backgroundImage="url(images/1.jpg)";
                break;
            case "2":
                document.body.style.backgroundImage="url(images/4.jpg)";
                break;
            case "3":
                document.body.style.backgroundImage="url(images/3.jpg)";
                break;
            case "4":
                document.body.style.backgroundImage="url(images/2.jpg)";
                break;
        }

    }

</script>